In the course of gas turbine engine development various dynamic concepts have evolved which require the mixing of two separate gas flows, such as bypass duct flow and exhaust gases, in an efficient manner. This mixing is required because the two gas flows are at widely varying temperatures and/or pressures and they must be combined together to form a single homogeneous flow of gases in order to reduce exhaust gas noise. A number of different noise suppression and gas mixing nozzles have been developed for use with aircraft gas turbine engines, some of which are independently attached to the engine around the engine exhaust duct, and some of which are combined with the engine exhaust duct. Nevertheless, the engine exhaust duct functions not only to discharge exhaust gases but also as a stationary support structure supporting for example, the turbine rotor bearing assembly. Therefore the engine exhaust duct is usually separated from the noise suppression and gas mixer in terms of either function or structure, and the engine exhaust duct with the noise suppression mixer usually requires a certain axial length for proper functioning.
Multi-lobed sound suppression shrouds are well known in the industry. Some of these are fabricated by a pressing process to form corrugations on a metal blank. One example of these is disclosed in U.S. Pat. No. 4,481,698, issued to Salerno on Nov. 13, 1984. Salerno employs a mechanical press with male and female die portions to form convolutions in a generally continuous circular ring blank. Other multi-lobed noise suppression mixers are fabricated by welding modular components together. For example, Stachowiak et al. disclose in their U.S. Pat. No. 4,335,801, issued on Jun. 22, 1982, a multi-lobe type noise suppressing nozzle having modular components welded together. This composite construction is provided to permit the addition of thickened lobe sidewall sections interconnected by inter-struts. The internal struts together with the thickened sidewall portions of the lobes define a structural ring providing hoop and bending strength at the center of the nozzle. The lobes of a multi-lobe type noise suppressing nozzle are generally unsupported and are subject to vibration and excessive deflections when in use. Therefore means for stiffening the lobes are usually required. For example, in U.S. Pat. No. 5,265,807, issued to Steckbeck et al. on Nov. 30, 1993, a circumferential stiffening ring is secured to the aft end of the mixer to reduce or prevent vibration while circumferentially enhancing the mixing of cool ambient air with the hot engine exhaust.
Modular components are also used to form other types of gas turbine mixers. For example, in U.S. Pat. No. 4,165,609, issued to Rudolph on Aug. 28, 1979, a tubular exhaust mixer includes a plurality of generally flat metal strips that extend axially and a plurality of vanes positioned between adjacent metal strips, in order to form first axially extending regions that induce fan air to flow radially inwardly, and second axially extending regions that induce primary exhaust gas to flow radially outwardly in order to penetrate the fan air stream. The two axially extending regions are circumferentially interspaced.
Although various types of engine exhaust ducts and noise suppression gas mixers have been developed, there is still a need for improved configurations of exhaust ducts and gas mixers. In particular, as gas turbine engines become smaller and approach general acceptance in General Aviation markets, there is now—more than ever—a need to provide gas turbine engines which are smaller, lighter, more reliable and, importantly, cheaper to produce and operate.